1. Field of the Invention
The present invention relates to a method for manufacturing a variator component of continuously variable automobile transmission, as well as to a variator component of the continuously variable transmission.
2. Background Art
In general, an input disk of a toroidal-type continuously variable transmission constitutes a ball spline disposed between the input disk and an input shaft to which rotation is transferred from the engine. The input disk can rotate in synchronism with the input shaft and can be moved in an axial direction of the input shaft. The ball spline is formed by means of placing ball spline grooves formed in the input shaft so as to oppose ball spline grooves formed in the input disk, and accommodating balls between the mutually-opposing ball spline grooves, whereby the input disk is engaged with the input shaft.
As mentioned above, the input disk is driven by way of the ball spline grooves formed in an inner radial region of the input disk and rotate around the center of the ball spline groove sections. Therefore, a toroidal surface and a cam surface, which are functional surfaces of the input disk, must be machined with high coaxiality with respect to the ball spline grooves which serve as a rotational reference. If the coaxiality between the functional surfaces and the ball spline grooves is low, contact of the toroidal surface with a power roller which transmits torque will become unstable, or pressing force developing in the cam surface will become unstable.
At the time of manufacture of the input disk, a blank is formed, through hot forging, into a general shape having a finished dimension with a machine allowance. The blank is then subjected to cutting work, whereby the blank is machined into a shape having an optimum allowance determined in consideration of heat processing distortion. In particular, in the case of a component having ball spline grooves formed in an inner radius region thereof, the shape and dimension of the inner radius region are formed through broaching.
After the blank formed through cutting work has been subjected to heat treatment and then cured, the ball spline grooves are subjected to finishing. A technique for chucking an outer radial surface of the blank as a machining reference and grinding the ball spline grooves one by one has been known as a finishing technique (see, e.g., JP-A-2000-233367).
In addition to this grinding technique, there has also been known a method for finishing the ball spline grooves through use of a hard broaching tool (see, e.g., U.S. Pat. Nos. 6,071,210 and 6,637,107). According to U.S. Pat. No. 6,637,107, a functional surface, such as a toroidal surface or a cam surface, is finished through grinding or turning while the finished ball spline grooves are taken as a machining reference.